Recently, semiconductor laser diodes have come to be widely used in a variety of types of electrical equipment, for example, laser printers, optical disk apparatuses, fiber-optic communication apparatuses, and mobile phones, because of their compact size, low cost, and ease of use.
However, the current/light-emission intensity characteristics of the semiconductor laser diode are dependent on temperature. Accordingly, it is necessary to control light-emission intensity to obtain a predetermined light-emission intensity reliably. This light-emission intensity control is called Automatic Power Control (APC). In the APC process, before the semiconductor laser diode is actually driven, the laser diode is driven in advance, quantity of light output from the laser diode is received by a photo diode (PD), and the detection current values of corresponding quantities of light are stored in a storage device. Then, the laser diode is controlled using the current values saved in the storage device so as to obtain a desired light-emission intensity reliably.
High resolution and high-speed operation are required of contemporary printers and copiers, including those that employ a semiconductor laser as a light source. In a case in which there is only a single laser beam used, in order to improve image resolution and printing speed it is necessary to increase modulation speed, which is the speed at which the semiconductor laser is driven (turned on and off) in accordance with the input image data. However, there is a limit to the modulation speed. Accordingly, in order to improve image resolution and operating speed without increasing the modulation speed, there is no alternative but to increase the number of laser beams.
In a case in which four laser beams are used, when it is assumed that the modulation speed and the printing speed are to the same as in a case in which a single beam is set in the laser light, the image resolution in the main-scanning direction and sub-scanning direction (horizontal and vertical directions) can be doubled. Alternately, in this case, when the image resolution is to the same as a case in which a single beam is set in the laser light, the printing speed can be quadrupled.
As for the semiconductor laser used for the light source, edge-emitting laser elements (hereinafter “edge-emitting lasers”) that emit laser light parallel to the activation layer are widely used. When the edge-emitting laser is used, the number of beams is set a single laser beam, or two or four multi-beam lasers is used in the printers and copiers. Since an optical axis between the lasers of the multi-beam laser is stable, when the multiple beams are required, it facilitates adjustment of the optical axis between the adjacent lasers by using the multi-beam laser rather than by adjusting multiple individual single lasers in the apparatus separately.
Generally, a laser unit of the edge-emitting laser includes a single photo-receiver element in addition to a multi-beam laser. The edge-emitting laser emits backward (back projection) proportional to a front projecting power as used as the laser light, and the photo-receiver element PD installed in the laser unit receives the back projection and generates a monitor current similarly proportional to the quantity of the light received. In the multi-beam laser, even when the powers of front projection for respective laser diodes are identical, the monitor currents thus generated are slightly different among the beams due to individual variability.
FIG. 1 illustrates a schematic diagram of a related art semiconductor laser driver 7 and control processes thereof. In this example, the semiconductor laser LD is constructed of multi-beam laser capable of outputting two beams from two laser diodes LD1 and LD2. The semiconductor laser driver 7 includes a semiconductor laser LD (multi-beam laser) emitting two laser beams, a light detection circuit 703 that detects the light-emission intensity from the semiconductor laser LD (LD1 and LD2), and the light-quantity control circuit 705 that controls light-emission intensities (quantity of light) of the laser diodes LD1 and LD2. The light detection circuit 703 includes a photoelectric conversion element (photodiode) PD, two variable resistors VR1 and VR2, and switches SW71 and SW72. The photo-receiver element PD functions as a photoelectric conversion element that converts the light-emission intensity outputted from the laser diodes LD1 and LD2 into electrical currents and outputs the converted currents as monitor current Im1 and Im2. The variable resistors VR1 and VR2 convert the monitor current Im input from the photo-receiver element PD into a voltage and output the converted voltage as a detection value Vdet of the light-emission intensity. The switches SW71 and SW72 connect and disconnect either of the variable resistor VR1 and VR2 corresponding to the laser diodes LD1 and LD2 to be adjusted.
Thus, the semiconductor laser driver 7 detects a detection voltage Vdet for the light-emission intensity Po of the laser diodes LD1 and LD2 by detecting the monitor currents Im1 and Im2 from the photo-receiver element PD that are almost proportional to the corresponding light-emission intensities of the laser diodes LD1 and LD2 by converting the monitor current Im by the variable resistors VR1 and VR2. Then, the semiconductor laser driver 7 controls the light-emission intensity Po so that the detection voltage Vdet of the light-emission intensity Po is set to a predetermined voltage based on a setting value Lset of light emission intensity.
In the adjustment of the light-emission intensity, while detecting a laser power indicating in a power meter 702 under APC by viewing (visual confirmation), an operator adjusts the resistance values Rvr of the variable resistors VR1 and VR2 such that the laser power is set to a predetermined laser power.
FIG. 2 is a diagram illustrating a light-current curve between a current-light feature of light-emission intensity Po of the semiconductor laser and a light-current feature of the monitor current Im of the semiconductor laser.
As shown in FIG. 2, the monitor current Im generated in the photo-deceiver element is almost proportional to the light-emission intensity Po. The detection voltage Vdet of the light-emission intensity is calculated by multiplying the monitor current Im by resistance values Rvr of the variable resistors VR1 and VR2 (Vset=Im×Rvr).
When the resistance value Rvr of the variable resistors VR1 and VR2 are decreased, the detection voltage Vdet of the light emission intensity is decreased, the light-emission intensity Po of the semiconductor laser LD is increased so as to increase (return) to the predetermined voltage to the detection voltage Vdet of the light emission, which increases the monitor current Im. That is, as the resistance values Rvr of the variable resistors VR1 and VR2 are decreased, the monitor current Im is controlled so that the light-emission intensity is increased. Conversely, as the resistance values Rvr of the variable resistors VR1 and VR2 are increased, the monitor current Im is controlled so that the light-emission intensity Po is decreased. With these adjusting processes, when the emission powers of the front projection of the beams from the laser diodes LD1 and LD2 in the multi-beam semiconductor laser LD are equal, respectively, the generated values of the monitor current Im are slightly different among the beams. In addition, even when the output powers of the quantities of light output from the respective laser beams are equal, emitting powers for photoreceptors in an image forming apparatus are different due to difference of transmissivity and reflectance of a lens constructing the laser unit of the semiconductor laser LD. That is, in order to set lighting power on the respective photoreceptors to be equal thereamong, it is required that the light-emission intensities of the respective laser beams are set separately.
Similarly, in a multicolor image forming apparatus, the receptive colors on the photoreceptors are assigned to multiple laser beams generated in the multi-beam laser LD, and powers for requiring the colors on the photoreceptor are set separately.
In this configuration, by setting respective variable resistors VR1 and VR2 under the state in which the setting value Lset of the light-emission intensity are set separately, a desired light-emission intensity can be attained.
Herein, the setting accuracy of the variable resistors is directly reflected to the adjustment accuracy of the light emission intensities. When the setting error of the variable resistors is 1%, the setting error of the light-emission intensity becomes 1%. Thus, since the resistance value Rvr is adjusted manually, the adjustment fluctuation occurs due to the error from the operator, which causes the quality of the light-emission intensity of the semiconductor laser LD to be degraded.
Further, although a rotation shaft of the variable resistor VR1 and VR2 are fixed by a resin to keep the resistance value Rvr after the resistance value Rvr thereof is manually adjusted, the rotation shaft may be moved in the market with time and vibration, which causes to change the resistance value Rvr thereof. Accordingly, the setting error of the light amount occurs, which causes the quality of the light-emission intensity of the semiconductor laser LD to be degraded. In JP-2008-227129-A proposes an adjusting method to adjust the resistance value Rvr not manually but using electrically controlling by controlling the multiple resistors to turn multiple analog switches on and off. In this example, the multiple resistors are discrete elements, and the analog switches and a control circuit for the analog switches are integrated onto a single chip (integrate circuit IC), the same number of terminals are required in the IC side corresponding to the number of the resistors. This resistor is required at least eight because at least 8 bit resolutions are required to fine-trim the resistance value. However, this configuration is not realistic in view of the number of the required terminals in IC. In addition, the ON-resistance of the analog switches cannot go ignored compared with outside resistance values.
In a configuration in which the resistor is integrated onto the single IC, the temperature fluctuation of the resistor becomes a problem. That is, the resistor is provided for compensating the temperature fluctuation of the light-emission intensity. When the temperature fluctuation of the monitor current corresponding to the quantity of light received from the photo-receiver element PD is set to zero and the temperature is changed by 50° C., in order to restrain the fluctuation of the light-emission intensity within 1%, it is necessary to keep the fluctuation of the light-emission intensity within 100 ppm/° C. (0.5%/50° C.=100 ppm/° C.). In the manufacture of the semiconductor laser driver, it is very difficult for normal active element formation processing to produce a resistor whose temperature fluctuation is within 100 ppm/° C. If special processing is added, the target tolerance can be achieved but at the cost of a manufacturing cost increase.